With Kim Jong Un turning up the heat with North Korea’s fourth nuclear test, South Korea responded Friday by pumping up the volume. Literally.

At noon on North Korean leader Kim’s birthday, South Korea fired up loudspeakers along the heavily fortified border and resumed the propaganda blasts that brought the reclusive regime to a war footing in August -- and then to the negotiating table. South Korea has reinforced defensive positions near the loudspeakers in case of attack, while the North Korean army has stepped up surveillance along the border, the Defense Ministry said.

A battery of propaganda loudspeakers along the border with North Korea

Photographer: Kim Jae-Hwan/AFP via Getty Images

While years of United Nations sanctions and other penalties have failed to bring Kim to heel, one thing that can get under his skin is broadcasts over the demilitarized zone of South Korean ballads and rap music, a genre known as K-pop. The speakers have been used only once in the past decade, for part of August in retaliation for the maiming of two South Korean soldiers by DMZ mines.

That spat escalated into what North Korea called a "semi-state of war" that was cooled by marathon talks at a border village where Kim’s officials agreed to halt the mobilization of forces. One condition was that Seoul turned the speakers off.

"Kim Jong Un isn’t your typical dictator. He’s a god in North Korea, and propaganda broadcasts raise questions among North Koreans about that,” said Park Chang Kwon, a senior research fellow at the state-run Korea Institute for Defense Analyses in Seoul. “Broadcasts from South Korea can reach deep and far into North Korea’s society, imbuing the minds of its people with the images of a free nation and hurting the oppressive personality cult.”

The broadcasts are a low-tech response to Kim’s saber-rattling, compared with options like the tightening of sanctions on the isolated regime, South Korea developing its own missile defense system or potentially a beefing up of the U.S. military presence south of the border. U.S. lawmakers are seeking tougher sanctions on North Korea while Secretary of State John Kerry has urged China to support a sterner approach to Kim’s regime.

August Pact

In parliament on Thursday, Defense Minister Han Min Koo said North Korea’s detonation of what it claims to be a hydrogen bomb constitutes an "abnormal situation." His words carried significance as the late August pact that ended the standoff could be annulled under such circumstances, allowing the loudspeakers to be turned back on.

The broadcasts challenge Kim’s monopoly on information. Speakers were set up at about 10 sites to play messages critical of North Korea’s political system along with songs by girl group Apink and folk singer Lee Ae Ran, the South Korean Defense Ministry said.

The August bursts ranged from K-Pop and recordings of casual conversations to discussions about the importance of human rights and the lives of South Korea’s middle class, according to the defense ministry in Seoul.

In October 2014, North Korea shot at balloons carrying anti-Pyongyang leaflets, and it has threatened artillery attacks against activists flying such materials over the border.

Map of the explosion site, the Punggye-ri test facility in North Korea, and the region

The loudspeaker move could yet backfire, given the level to which the propaganda irritates the regime.

“The resumption of loudspeaker broadcasts may emotionally provoke the North Korean military sensitive to criticism of the ‘supreme dignity,’ rather than help resolve the nuclear issue,” Cheong Seong Chang, a senior researcher at the Sejong Institute near Seoul, said by text message.

With Kim’s birthday -- he is believed to be in his early thirties -- falling on the day of the new broadcasts, “North Korea may react in an ultra-strong way to this decision by South Korea, viewing it as an act of ruining a national party.”